How the Neuron Sprouts Its Branches

Neurobiologists have gained new insights into how neurons control growth of the intricate tracery of branches called dendrites that enable them to connect with their neighbors. Dendritic connections are the basic receiving stations by which neurons form the signaling networks that constitute the brain’s circuitry.

Such basic insights into neuronal growth will help researchers better understand brain development in children, as well as aid efforts to restore neuronal connections lost to injury, stroke or neurodegenerative disease, said the researchers.

In a paper published in the Dec. 8, 2005, issue of Neuron, Howard Hughes Medical Institute investigator Michael Ehlers and his colleagues reported that structures called “Golgi outposts” play a central role as distribution points for proteins that form the building blocks of the growing dendrites.

Besides Ehlers, who is at Duke University Medical Center, other co-authors were April Horton in Ehlers’ laboratory; Richard Weinberg of the University of North Carolina at Chapel Hill.; Bence Rácz in Weinberg’s laboratory; and Eric Monson and Anna Lin of Duke’s Department of Physics. The research was sponsored by The National Institutes of Health.

The Golgi apparatus is a cellular warehouse responsible for receiving, sorting and shipping cargoes of newly synthesized molecules needed for cell growth and function. Until the new findings, researchers believed that only a central Golgi apparatus played a role in such distribution, said Ehlers.

“In most mammalian cells, the Golgi has a very stereotyped structure, a stacked system that resides near the cell nucleus in the middle of the cell,” he said. “But mammalian neurons in the brain are huge, with a surface area about ten thousand times that of the average cell. So, it was an entirely open question where all the membrane components came from to generate the complex surface of growing dendrites. And we thought these remote structures we had discovered in dendrites called Golgi outposts might play a role.”

The researchers studied the dendritic growth process in pyramidal neurons, which grow a single long “apical” dendrite and many shorter ones. To explore the role of Golgi outposts, they used imaging of living rat brain cells grown in culture, as well as electron microscopy of rat brain tissue.

These studies revealed that the Golgi outposts tended to appear in longer dendrites and also that those Golgi in the main cell body tended to orient toward longer dendrites. And importantly, said Ehlers, the studies in cell culture revealed that the Golgi orientation preceded the preferential growth of long dendrites.

“This finding showed us that we weren’t just seeing a correlation between Golgi and longer dendrites,” said Ehlers. “Initially, when these growing dendrites are all essentially uniform in length, they grow at about the same rate. But later, after the Golgi orient toward one dendrite, it takes off and grows dynamically to become the longest dendrite.” The researchers also used tracer molecules to track the molecular cargo secreted by the Golgi, said Ehlers.

“We saw very clearly that this cargo that originates in the Golgi gets directed towards the one longest dendrite in a highly preferential way,” he said. “As cargo comes out of the Golgi, it does not go randomly to the cell surface.” Ehlers and his colleagues also found that the Golgi outposts appeared to locate themselves at dendritic branch points.

“This finding is important because a fundamental problem that neurons must solve is how to sort appropriate cargo molecules in the right amounts down different dendritic branches,” said Ehlers. “After all, different dendritic branches can have different functional properties, molecular composition and electrical properties. So, when a cargo reaches a branch point, it’s like a highway intersection, and the cargo needs to be directed. We’ve found that these dendritic Golgi outposts are located at the strategic points to do just that. And I believe this is the first such specific organelle identified at a dendritic branch point positioned to perform this fundamental neuronal function.”

Finally, the researchers disrupted the orientation, or “polarity,” of the Golgi — thus causing them to move into all the dendrites — without disrupting their function. They found that disrupting the polarity caused all the dendrites to grow at the same rate.

Further studies, said Ehlers, will explore how Golgi outposts arise, how they arrive at dendritic branch points and what cargo they distribute. The researchers also will seek to understand how molecules are selected for import to the distant reaches of the dendrites and which will be locally synthesized in the dendrites. Such studies could give important insights into the machinery of neuronal growth and how it is controlled, he said.

“Understanding this machinery has clinical relevance because many disorders of brain development in children manifest abnormal dendritic structures,” said Ehlers. “Also, it turns out that most neurodegenerative diseases, such as Parkinson’s and Alzheimer’s, are disorders of protein processing. But we know very little about how and where integral membrane proteins are synthesized and processed by neurons.”